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The   Theory  of  Drying  and  its  Application  to   the  Hew 
Humid ity-Heg Plated  and  Reeirculating  Dry  Kilns 


aui 


UNITED  STATES  DEPARTMENT  OF  AGRICULTURE 

BULLETIN  No.  509 


Contribution  from  the  Forest  Service 
HENRY  S.  GRAVES,  Forester 


Washington,  D.  C. 


PROFESSIONAL  PAPER 


March  17,  1917 


THE  THEORY  OF  DRYING  AND  ITS  APPLICATION 
TO  THE  NEW  HUMIDITY-REGULATED  AND  RE- 
CIRCULATING  DRY  KILN. 

By  HABRY  D.  TIEMANN,  In  Charge,  Section  of  Timber  Physics,  Forest  Products 
' '  Laboratory. 


CONTENTS. 


Introduction 

Elementary  principles  of  drying 

Elementary  principles  of  hygrometry. 

Types  of  kilns 

Drying  by  superheated  steam 


Page. 
1 
2 
5 
7 
8 


Page. 

Importance  of  proper  piling  of  lumber 9 

Theory  and  description  of  the  Forest  Service 
kirn 10 

Theoretical  discussion  of  evaporation 13 

Theoretical  analysis  of  heat  quantities 18 


INTRODUCTION. 

The  problem  of  satisfactorily  drying  lumber  without  checking, 
honeycombing,  or  warping  is  one  of  very  wide  interest.  Although 
an  old  problem,  it  has  not  yet  reached  an  entirely  satisfactory  solu- 
tion, especially  with  hardwood  lumber.  Even  air  drying,  which  is 
the  slowest  and  what  might  be  called  the  most  conservative  method 
of  removing  the  moisture,  is  far  from  satisfactory  for  some  species  of 
wood.  The  drying  of  softwoods,  or  wood  from  coniferous  trees, 
on  the  other  hand,  may  be  considered  as  having  reached  a  fairly  sat- 
isfactory solution.  With  few  exceptions,  the  softwoods  present  no 
special  difficulty  to  the  lumber  drier.  The  great  trouble  with  the 
hardwoods  lies  in  their  relatively  excessive  and  very  unequal  shrink- 
age. This  is  due  largely  to  the  structure  of  the  wood.  In  soft- 
woods the  vertical  elements  are  all  of  the  same  kind,  regularly  ar- 
ranged and  of  approximately  the  same  width  (tangentially).  The 
medullary  rays  also  are  very  fine  and  regular.  In  hardwoods,  on  the 
other  hand,  the  elements  are  very  complex,  varying  in  diameter  in 
some  species  in  the  same  section  20  to  30  times,  and  are  often  very 
crooked.  Many  woods,  such  as  the  oak,  have  large  medullary  rays,  as 

70253°— Bull.  509—17 1 


437650 


3-7 


2        ••:BtrLLETOjF:5p9;  ii.-s.  DEPARTMENT  OF  AGRICULTURE. 


^iS  yej-r  £mffftonesvirfegularly  arranged.  Consequently,  strains 
are;pro<ftrcecf  when:  thfc'  wbbcl  dries,  which  cause  warping  and  check- 
ing. While  air  drying  is  undoubtedly  the  safest  method,  the  process 
is  ordinarily  so  slow,  requiring  a  year  or  longer  according  to  species 
and  size,  that  forced  "artificial"  drying  becomes  a  business  neces- 
sity. Moreover,  air  drying  is  by  no  means  always  to  be  preferred  to 
kiln  drying  from  the  standpoint  of  the  quality  of  the  product. 

A  correct  understanding  of  the  principles  of  drying  is  rare,  and 
opinions  in  regard  to  the  subject  are  very  diverse.  The  same  lack  of 
knowledge  exists  in  regard  to  dry  kilns.  The  physical  properties 
of  the  wood  which  complicate  the  drying  operation  and  render  it 
distinct  from  that  of  merely  evaporating  free  water  from  some  sub- 
stance like  a  piece  of  cloth  must  be  studied  experimentally.  It  can 
not  well  be  worked  out  theoretically. 

The  thermal  process  of  the  drying  operation,  however,  is  capable 
of  exact  theoretical  analysis.  It  is  the  purpose  of  this  article  to 
interpret  the  conditions  which  exist  in  the  various  stages  of  the  dry- 
ing operation  with  respect  to  the  heat  quantities  and  the  changes 
which  occur  in  the  drying  medium,  from  a  theoretical  standpoint. 
The  object  of  this  analysis  is  to  show  the  limiting  conditions  which 
may  be  approached,  but  can  not  be  exceeded. 

ELEMENTARY  PRINCIPLES  OF  DRYING. 

Before  taking  up  the  theoretical  discussion,  a  few  remarks  upon  the 
elementary  principles  of  drying  will  be  of  assistance. 

EVAPORATION  REQUIRES  HEAT. 

In  the  'first  place,  it  should  be  borne  in  mind  that  it  is  the  heat 
which  produces  evaporation  and  not  the  air  nor  any  mysterious 
property  assigned  to  a  "  vacuum."  For  every  pound  of  water  evapo- 
rated at  ordinary  temperatures  approximately  1,000  British  thermal 
units  of  heat  are  used  up,  or  "  become  latent,"  as  it  is  called.  This 
is  true  whether  the  evaporation  takes  place  in  a  vacuum  or  under  a 
moderate  air  pressure.  If  this  heat  is  not  supplied  from  an  outside 
source  it  must  be  supplied  by  the  water  itself  (or  the  body  being 
dried),  the  temperature  of  which  will  consequently  fall  until  the  sur- 
rounding space  becomes  saturated  with  vapor  at  a  pressure  cor- 
responding to  the  temperature  which  the  water  has  reached  ;  evapora- 
tion will  then  cease.  The  pressure  of  the  vapor  in  a  space  saturated 
with  water  vapor  increases  rapidly  with  increase  of  temperature. 
At  a  so-called  vacuum  of  28  inches,  which  is  about  the  limit  in  com- 
mercial operations,  and  in  reality  signifies  an  actual  pressure  of  2 
inches  of  mercury  column,  the  space  will  be  saturated  with  vapor 
at  about  101°  F.  Consequently,  no  evaporation  will  take  place  in 
such  a  vacuum  unless  the  water  be  warmer  than  101°  F.,  provided 


HUMIDITY-REGULATED  AND  RECIRCULATING   DRY   KILN.  3 

there  is  no  air  leakage.  The  qualification  in  regard  to  air  is  neces- 
sary, for  the  sake  of  exactness,  for  the  following  reason :  In  any  given 
space  the  total  actual  pressure  is  made  up  of  the  combined  pressures 
of  all  the  gases  present.  If  the  total  pressure  ("vacuum")  is  2 
inches,  and  there  is  no  air  present,  it  is  all  produced  by  the  water 
vapor  (which  saturates  the  space  at  101°  F.) ;  but  if  some  air  is  pres- 
ent and  the  total  pressure  is  still  maintained  at  2  inches,  then  there 
must  be  less  vapor  present,  since  the  air  is  producing  part  of  the 
pressure  and  the  space  is  no  longer  saturated  at  the  given  tempera- 
ture. Consequently  further  evaporation  may  occur,  with  a  cor- 
responding lowering  of  the  temperature  of  the  water,  until  a  balance 
is  again  reached.  Without  further  explanation  it  is  easy  to  see  that 
but  little  water  can  be  evaporated  by  a  vacuum  alone  without  addi- 
tion of  heat  and  that  the  prevalent  idea  that  a  vacuum  can  of  itself 
produce  evaporation  is  a  fallacy.  If  heat  be  supplied  to  the  water, 
however,  either  by  conduction  or  radiation,  evaporation  will  take 
place  in  direct  proportion  to  the  amount  of  heat  supplied,  so  long  as 
the  pressure  is  kept  down  by  the  pump. 

At  30  inches  of  mercury  pressure  (one  atmosphere)  the  space  be- 
comes saturated  with  vapor  and  equilibrium  is  established  at  212°  F. 
If  heat  be  now  supplied  to  the  water,  however,  evaporation  will  take 
place  in  proportion  to  the  amount  of  heat  supplied,  so  long  as  the 
pressure  remains  that  of  one  atmosphere,  just  as  in  the  case  of  the 
vacuum.  Evaporation  in  this  condition,  where  the  vapor  pressure 
at  the  temperature  of  the  water  is  equal  to  the  gas  pressure  on  the 
water,  is  what  is  commonly  called  "  boiling,"  and  the  saturated  vapor 
entirely  displaces  the  air  under  continuous  operation.  Whenever 
the  space  is  not  saturated  with  vapor,  whether  air  is  present  or  not, 
evaporation  will  take  place,  by  boiling  if  no  air  be  present  or  by 
diffusion  under  the  presence  of  air,  until  an  equlibrium  between 
temperature  and  vapor  pressure  is  resumed. 

Relative  humidity  is  simply  the  ratio  of  the  actual  vapor  pres- 
sure present  in  a  given  space  to  the  vapor  pressure  when  the  space 
is  saturated  with  vapor  at  the  given  temperature.  It  matters  not 
whether  air  be  present  or  not.  One  hundred  per  cent  humidity 
means  that  the  space  contains  all  the  vapor  which  it  can  hold  at  the 
given  temperature— it  is  saturated.  Thus  at  100  per  cent  humidity 
and  212°  F.  the  space  is  saturated,  and  since  the  pressure  of  satu- 
rated vapor  at  this  temperature  is  one  atmosphere,  no  air  can  be 
present  under  these  conditions.  If,  however,  the  total  pressure  at 
this  temperature  were  20  pounds  (5  pounds  gauge),  then  it  would 
mean  that  there  was  5  pounds  air  pressure  present  in  addition  to  the 
vapor,  yet  the  space  would  still  be  saturated  at  the  given  tempera- 
ture. Again,  if  the  temperature  were  101°  F.,  the  pressure  of  satu- 
rated vapor  would  be  only  1  pound,  and  the  additional  pressure  of 


4  BULLETIN  509,  U.  S.  DEPARTMENT  OF  AGRICULTURE. 

14  pounds,  if  the  total  pressure  were  atmospheric,  would  be  made  up 
of  air.  In  order  to  have  no  air  present  and  the  space  still  satu- 
rated at  101°  F.,  the  total  pressure  must  be  reduced  to  1  pound  by  a 
vacuum  pump.  'Fifty  per  cent  relative  humidity,  therefore,  signifies 
that  only  half  the  amount  of  vapor  required  to  saturate  the  space  at 
the  given  temperature  is  present.  Thus  at  212°  F.  temperature  the 
vapor  pressure  would  only  be  7J  pounds  (vacuum  of  15  inches  gauge). 
If  the  total  pressure  were  atmospheric,  then  the  additional  TJ  pounds 
is  simply  air.  "  Live  steam "  is  simply  saturated  water  vapor  at  a 
pressure  usually  above  atmospheric.  We  may  just  as  truly  have  live 
steam  at  pressures  less  than  atmospheric,  at  a  vacuum  of  28  inches  for 
instance.  Only  in  the  latter  case  its  temperature  would  be  lower, 
viz,  101°  F.  Superheated  steam  is  nothing  more  than  water  vapor 
at  a  relative  humidity  less  than  saturation,  but  is  usually  considered 
at  pressures  above  atmospheric,  and  in  the  absence  of  air.  The 
atmosphere  at,  say,  50  per  cent  relative  humidity  really  contains 
superheated  steam  or  vapor,  the  only  difference  being  that  it  is  at 
a  lower  pressure  and  temperature  than  we  are  accustomed  to  think 
of  in  speaking  of  superheated  steam,  and  it  has  air  mixed  with  it  to 
make  up  the  deficiency  in  pressure  below  the  atmosphere. 

Two  things  should  now  be  clear :  That  evaporation  is  produced  by 
heat  and  that  the  presence  or  absence  of  air  does  not  influence  the 
amount  of  evaporation.  It  does,  however,  influence  the  rate  of 
evaporation,  which  is  retarded  by  the  presence  of  air.  The  main 
things  influencing  evaporation  are,  first,  the  quantity  of  heat  sup- 
plied and,  second,  the  relative  humidity  of  the  immediately  sur- 
rounding space. 

IMPORTANCE  OF  CIRCULATION. 

A  piece  of  wood  may  be  heated  in  three  ways — (1)  by  convection 
of  the  air  and  vapor  or  other  gases,  (2)  by  conduction  through  some 
body  in  contact  therewith,  and  (3)  by  radiation.  Of  these  three 
ways,  only  the  first  is  ordinarily  available  for  use  in  heating  a  pile 
of  lumber,  since  by  either  of  the  other  two  methods  only  the  outside 
surface  of  the  pile  could  be  heated;  hence  the  necessity  of  a  large 
and  thorough  circulation  of  air.  Drying  in  a  vacuum  would  be 
feasible  if  there  were  some  means  of  conveying  the  heat  to  the  wood. 
A  single  stick  can  be  readily  dried  in  a  vacuum,  as  it  can  receive 
heat  on  all  sides  by  radiation  from  the  walls  of  a  steam- jacketed 
cylinder;  but  this  is  impracticable  when  it  comes  to  any  quantity  of 
lumber,  except  in  the  case  of  superheated  vapor  alone,  as  will  be 
shown  later,  since  only  the  outer  surface  or  the  outside  boards  would 
receive  the  heat  in  this  way  and  the  inside  ones  would  not  dry. 
Even  an  approach  to  a  perfect  vacuum,  however,  is  not  reached  in 
commercial  apparatus.  Moreover,  the  heat  convection  in  a  vacuum 


HUMIDITY-REGULATED  AND  RECIBCULATING  DRY  KILN.  5 

of  26  inches  or  less  is  almost  as  rapid  as  under  ordinary  air  pressure.1 
•  The  viscosity  of  the  gas  is  a  factor  in  the  convection  through  small 
spaces,  such  as  between  the  layers  of  lumber,  and  as  this  is  almost 
i  as  great  at  low  pressures  as  at  atmospheric  pressure,  it  follows  that 
the  actual  circulation  would  nevertheless  be  very  much  cut  down. 
Thus,  by  drawing  a  vacuum  the  means  of  heating  the  wood  is  re- 
duced.    Later  on  it  will  be  shown,  however,  that  drying  at  low 
pressure  in  absence  of  air  should  give  the  highest  theoretical  heat 
efficiency,  but  the  volume  of  vapor  required  is  excessive. 

RATE  OF  EVAPORATION  CONTROLLED  BY  HUMIDITY. 

It  is  essential,  therefore,  to  have  an  ample  supply  of  heat  through 
the  convection  currents  of  the  air;  but  in  the  case  of  wood  the  rate 
of  evaporation  must  be  controlled,  else  checking  will  occur.  This  can 
be  done  by  means  of  the  relative  humidity.  It  is  clear  now  that 
when  the  air — or,  more  properly  speaking,  the  space — is  completely 
saturated  no  evaporation  can  take  place  at  the  given  temperature. 
By-  reducing  the  humidity,  evaporation  takes  place  more  and  more 
rapidly. 

Another  bad  feature  of  an  insufficient  and  nonuniform  supply  of 
heat  is  that  each  piece  of  wood  will  be  heated  to  the  evaporating 
point  on  the  outer  surface,  the  inside  remaining  cool  until  consider- 
able drying  has  taken  place  from  the  surface.  Ordinarily  in  dry 
kilns  high  humidity  and  large  circulation  of  air  are  antitheses  to 
one  another.  To  obtain  the  high  humidity  the  circulation  is  either 
stopped  altogether  or  greatly  reduced,  and  to  reduce  the  humidity  a 
greater  circulation  is  induced  by  opening  the  ventilators  or  otherwise 
increasing  the  draft.  This  is  evidently  not  good  practice,  but  as 
a  rule  is  unavoidable  in  most  kilns.  The  humidity  should  be  raised 
to  check  evaporation  without  reducing  the  circulation. 

ELEMENTARY  PRINCIPLES  OF  HYGROMETRY. 

RELATIVE  HUMIDITY  AND  DEW  POINT. 

It  is  necessary  to  know  something  of  hygrometry  in  order  to  under- 
stand the  drying  operations.  As  stated  before,  at  any  given  tempera- 
ture the  same  quantity  of  water  vapor  is  required  to  saturate  a  given 

1  Bottomly  gives  for  radiation  of  a  bright  platinum  wire  to  a  copper  envelope,  at  differ- 
ent air  pressures,  the  temperature  of  the  inclosure  being  16°  C.  and  the  difference  in 
temperature  408°  C.  expressed  in  the  heat  lost  in  c.  g.  s.  units  per  square  centimeter  of 
inclosure  (Smithsonian  Table  250)  : 

At  740  mm.  absolute  pressure 0.  8137 

At  42  mm.  absolute  pressure .  7591 

At  0.44  mm.  absolute  pressure .  2683 

At  0.01  mm.  absolute  pressure .0539 

These  figures  evidently  include  radiation  and  convection.  They  show  comparatively 
small  change  at  pressures  above  42  millimeters  of  mercury,  which  corresponds  to  a 
vacuum  of  about  28.4  inches. 


6  BULLETIN  509,  U.  S.  DEPARTMENT  OF  AGRICULTURE. 

space,  whether  any  air  is  present  or  not;  and  the  pressure  of  the 
vapor  is  the  same  in  both  cases.  The  total  pressure  (as  registered  by 
the  gauge)  will  not  be  the  same,  however,  since  if  air  is  present  its 
pressure  is  added  to  that  of  the  vapor.  It  is  really  the  space  and 
not  the  air  which  is  saturated.  For  instance,  at  101°  F.  it  takes  about 
20  grains  of  vapor  to  saturate  a  cubic  foot  of  space.  If  no  air  be 
present,  there  will  be  a  pressure  of  vapor  only,  which  will  be  about 
1  pound,  or  a  vacuum  of  28  inches.  If  this  is  open  to  the  atmosphere 
the  air  will  rush  into  the  space  until  the  total  pressure  will  be  one 
atmosphere,  or  about  15  pounds.  There  will  then  be  1  pound  of  pres- 
sure produced  by  the  vapor,  as  before,  and  14  pounds  of  air  pressure. 
The  space  will  still  be  saturated,  if  the  temperature  is  kept  at  101°  F. 
If  this  is  now  heated  to  160°  F.  and  open  to  the  atmosphere  so  that 
the  total  pressure  is  kept  constant,  the  ratio  of  the  pressures  of  vapor 
and  air  will  also  remain  the  same ;  there  will  still  be  1  pound  due  to 
vapor  and  14  pounds  due  to  the  air.  (The  weights  in  the  cubic  foot 
of  space  of  both  will  decrease,  due  to  expansion  by  heat.)  At  160°  Ft, 
however,  it  requires  91  grains  of  vapor  to  saturate  a  cubic  foot  of 
space,  and  its  pressure  is  nearly  5  pounds  (absolute).  Consequently, 
the  relative  humidity  at  160°  F.  of  this  space  will  be  one-fifth,  or  20 
per  cent.  Conversely,  if  this  air  and  vapor  at  20  per  cent  relative 
humidity  and  160°  F., temperature  is  cooled  to  101°  F.,  all  at  the  same 
atmospheric  pressure,  the  space  will  again  become  saturated,  and  any 
further  cooling  will  cause  precipitation  or  condensation.  This  is 
called  the  dew  point;  that  is,  101°  F.  is  the  dew  point  of  air  with  20 
per  cent  humidity  at  160°  F.  In  Forest  Service  Bulletin  104,  "  Prin- 
ciples of  Drying  Lumber  at  Atmospheric  Pressure  and  Humidity 
Diagram,"  a  humidity  diagram  is  given  for  solving  all  problems  of 
this  nature.  The  concave  curves  on  this  diagram  are  simply  curves 
of  constant  vapor  pressure  with  change  of  temperature  and  relative 
humidity,  and  the  grains  of  vapor  per  cubic  foot,  at  saturation  or  -the 
dew  point,  are  given  in  numerical  figures.  From  this  it  is  seen  that 
the  dew  point  determines  the  relative  humidity  when  the  temperature 
is  raised,  or  vice  versa.  If  we  take  saturated  air  at  known  tempera- 
ture and  heat  it  up  any  given  desired  amount,  the  resulting  relative 
humidity  is  thereby  determined.  This  is  the  principle  upon  which 
the  humidity  regulation  depends  in  a  new  kiln  designed  by  the 
writer.1  It  is  also  evident  that  whenever  air  is  cooled  below  its  dew 
point  condensation  takes  place.  This  is  the  principle  of  the  con- 
denser. There  are  a  number  of  kilns  which  have  made  use  of  this 
principle  to  dry  the  air.  Pipes  are  used  for  the  condensers  and  cold 
water  is  circulated  through  the  pipes.  The  same  thing  can  be  accom- 
plished by  a  spray  of  cold  water  in  place  of  the  pipes,  provided  all 
the  fine  mist  is  subsequently  removed  from  the  air,  or  even  by  a  sur- 

1  For  a  description  of  this  kiln  see  page  10. 


HUMIDITY-REGULATED  AND  RECIRCULATING  DRY  KILN.  7 

face  of  cold  water.  In  the  new  kiln  a  fine  spray  of  water  is  used  in- 
stead of  a  condenser.  This  has  the  additional  advantage  that  when 
the  water  is  heated  above  a  certain  temperature  (the  temperature  of 
the  wet  bulb  in  a  wet-and-dry  bulb  hygrometer)  it  will  humidify  the 
air.  By  simply  changing  the  temperature  of  the  spray  the  air  may 
be  supplied  at  any  desired  humidity. 

INSTRUMENTS  FOR  MEASURING  HUMIDITY. 

A  common  instrument  used  for  measuring  humidity  is  the  wet- 
and-dry  bulb  hygrometer  or  "  psychrometer."  This  consists  of  two 
thermometers  mounted  side  by  side,  the  bulb  of  one  of  which  is 
covered  by  a  silk  cloth  or  wick  which  dips  into  a  vessel  of  water. 
This  should  be  placed  in  a  fairly  strong  draft  of  air.  The  evapora- 
tion from  the  "  wet  bulb  "  reduces  its  temperature  below  that  of  the 
dry  bulb,  and  the  rate  of  this  evaporation,  and  consequently  the 
temperature  of  the  wet  bulb,  depends  upon  the  relative  humidity  in 
the  air.  By  noting  the  two  temperatures  of  the  dry  and  the  wet 
bulb  thermometers  the  relative  humidity  can  be  determined  by  tables 
which  have  been  carefully  worked  out  by  the  Weather  Bureau.1 

In  the  humidity  diagram  in  Forest  Service  Bulletin  104  the  values 
are  expressed  in  curves  (the  convex  series  of  curves  on  the  diagram) , 
by  means  of  which  the  relative  humidity  may  be  read  off  directly 
without  numerical  calculations.  This  instrument  is  probably  the 
simplest  reliable  means  for  determining  humidity.  There  are  instru- 
ments which  read  directly  from  a  hand  on  a  dial,  the  motion  of  the 
hand  being  produced  by  the  swelling  of  vegetable  or  animal  tissues. 
These  are  very  convenient  but  fragile  and  not  to  be  depended  upon. 
The  most  direct  way  of  determining  humidity  is,  of  course,  to  de- 
termine the  dew  point.  This  may  be  accomplished  by  gradually 
cooling  a  bright  surface,  as  polished  metal,  in  contact  with  the  mov- 
ing air,  until  a  mist  is  precipitated  thereon.  Special  interest  attaches 
to  the  wet-and-dry  bulb  hygrometer  for  the  reason  that  the  wet  wood 
in  the  dry  kiln  is  actually  in  the  same  condition  as  the  wet  bulb.  It 
is  affected  in  the  same  way.  The  actual  temperature  of  the  wood, 
while  it  is  moist,  is  therefore  that  of  the  wet  bulb,  provided  there  is 
sufficient  circulation. 

TYPES  OF  KILNS. 

There  are  two  distinct  ways  of  handling  lumber  in  kilns.  One 
way  is  to  place  the  load  of  lumber  in  a  chamber  where  it  remains  in 
the  same  place  throughout  the  operation,  while  the  conditions  of  the 
drying  medium  are  varied  as  the  drying  progresses.  This  is  the 
compartment  kiln  or  stationary  method.  The  other  is  to  run  the 
lumber  in  one  end  of  the  chamber  on  a  wheeled  truck  and  gradually 

1  See  Psychrometer  Tables  by  Marvin,  Bulletin  235  of  United  States  Weather  Bureau. 


8  BULLETIN   509,  U.  S.  DEPARTMENT  OF  AGRICULTURE. 

move  it  along  until  the  drying  process  is  completed,  when  it  is  taken 
out  at  the  opposite  end  of  the  kiln.  An  attempt  is  usually  made  in 
these  kilns  to  maintain  one  end  moist  and  the  other  end  dry.  This  is 
known  as  the  "  progressive  "  type  of  kiln,  and  is  the  one  most  com- 
monly used  in  large  operations.  It  is  the  least  satisfactory  of  the  two, 
however,  where  careful  drying  is  required,  since  the  conditions  can 
not  be  so  well  regulated  and  the  temperatures  and  humidities  are  apt 
to  change  with  change  of  wind.  The  compartment  method  can  be 
arranged  so  that  it  will  not  require  any  more  kiln  space  or  any  more 
handling  of  lumber  than  the  progressive  type.  It  does,  however, 
require  more  intelligent  operation,  since  the  conditions  in  the  kiln 
must  be  changed  as  the  drying  progresses.  With  the  progressive 
type  the  conditions,  once  established,  remain  the  same. 

To  obtain  draft  or  circulation  three  methods  are  in  use — by  forced 
draft  or  a  blower  usually  placed  outside  the  kiln,  by  ventilation,  and 
by  internal  circulation  and  condensation.  A  great  many  patents  have 
been  taken  out  on  different  methods  of  ventilation,  but  in  actual 
operation  few  work  exactly  as  intended.  Frequently  the  air  moves 
in  the  reverse  direction  for  which  the  ventilators  were  planned. 
Sometimes  a  condenser  is  used  in  connection  with  the  blower  and 
the  air  is  recirculated.  It  is  also — and  more  satisfactorily — used 
with  the  gentle  internal-gravity  currents  of  air. 

Many  patents  have  been  taken  out  for  heating  systems.  The  differ- 
ences among  these,  however,  have  more  to  do  with  the  mechanical 
construction  than  with  the  process  of  drying.  In  general,  the  heating 
is  either  direct  or  indirect.  In  the  former  steam  coils  are  placed  in 
the  chamber  with  the  lumber,  and  in  the  latter  the  air  is  heated  by 
either  steam  coils  or  a  furnace  before  it  is  introduced  into  the  kiln. 

Moisture  is  sometimes  supplied  by  means  of  free  steam  jets  in  the 
kiln  or  in  the  entering  air;  but  more  often  the  moisture  evaporated 
from  the  lumber  is  relied  upon  to  maintain  the  humidity  necessary. 
In  the  new  humidity-regulated  kiln  the  humidity  is  controlled  di- 
rectly. The  majority  of  kilns  make  no  attempt  whatever  to  regulate 
this  all-important  factor  beyond  retaining  an  indeterminate  amount 
at  the  beginning  of  the  operation  and  drying  the  air,  either  by  con- 
densers or  by  ventilation  at  the  end. 

Other  methods  of  drying  in  vacuum  and  in  various  gases  have  been 
tried  from  time  to  time. 

DRYING  BY  SUPERHEATED  STEAM. 

There  is  still  another  type  of  kiln  which  is  not  included  in  the 
former  classification,  viz,  that  using  superheated  steam.  What  this 
term  really  signifies  is  simply  water  vapor  in  the  absence  of  air  in  a 
condition  of  less  than  saturation.  Such  kilns  are,  properly  speak- 
ing, vapor  kilns,  and  usually  operate  at  atmospheric  pressure,  but 


HUMIDITY-REGULATED  AND  RECIRCULATING   DRY   KILN.  9 

may  be  used  at  greater  pressures  or  at  less  pressures.  As  stated 
before,  the  vapor  present  in  the  air  at  any  humidity  less  than  satura- 
tion is  really  "  superheated  steam,"  only  at  a  lower  pressure  than  is 
ordinarily  understood  by  this  term,  and  mixed  with  air.  The  main 
argument  in  favor  of  this  process  seems  to  be  based  on  the  idea  that 
steam  is  moist  heat.  This  is  true,  however,  only  when  the  steam  is 
near  saturation.  When  it  is  superheated  it  is  just  as  dry  as  air  con- 
taining the  same  relative  humidity.  For  instance,  steam  at  atmos- 
pheric pressure  and  heated  to  248°  F.  has  a  relative  humidity  of 
only  50  per  cent  and  is  just  as  dry  as  air  containing  the  same  hu- 
midity. If  heated  to  306°  F.,  its  relative  humidity  is  reduced  to  20 
per  cent;  that  is  to  say,  the  ratio  of  its  actual  vapor  pressure  (one 
atmosphere)  to  the  pressure  of  saturated  vapor  at  this  temperature 
(five  atmospheres)  is  1 : 5,  or  20  per  cent.  Superheated  vapor  in  the 
absence  of  air,  however,  parts  with  its  heat  with  great  rapidity  and 
finally  becomes  saturated  when  it  has  lost  all  of  its  ability  to  cause 
evaporation.  In  this  respect  it  is  more  moist  than  air  when  it  comes 
in  contact  with  bodies  which  are  at  a  lower  temperature.  When  sat- 
urated steam  is  used  to  heat  the  lumber  it  can  raise  the  temperature 
of  the  latter  to  its  own  temperature,  but  can  not  produce  evapora- 
tion unless,  indeed,  the  pressure  is  varied.  Only  by  the  heat  supplied 
aboA~e  the  temperature  of  saturation  can  evaporation  be  produced. 
This  subject  will  be  taken  up  again  in  the  theoretical  analysis. 

IMPORTANCE  OF  PROPER  PILING  OF  LUMBER. 

The  efficiency  of  the  drying  operation  depends  a  great  deal  upon 
the  way  in  which  the  lumber  is  piled,  especially  when  the  humidity 
is  not  regulated.  From  the  theory  of  drying  just  discussed  it  is 
evident  that  the  rate  of  evaporation  in  kilns  where  the  humidity  is 
not  regulated  depends  entirely  upon  the  rate  of  circulation,  other 
things  being  equal.  Consequently,  those  portions  of  the  wood  which 
receive  the  greatest  amount  of  air  dry  the  most  rapidly,  and  vice 
versa.  The  only  way,  therefore,  in  which  anything  like  uniform 
drying  can  take  place  is  where  lumber  is  so  piled  that  each  portion 
of  it  comes  in  contact  with  the  same  amount  of  air. 

In  the  Forest  Service  kiln,  where  the  degree  of  relative  humidity 
is  used  to  control  the  rate  of  drying,  the  amount  of  circulation 
makes  little  difference,  provided  it  exceeds  a  certain  amount.  It  is 
desirable  to  pile  the  lumber  so  as  to  offer  as  little  frictional  resist- 
ance as  possible  and  at  the  same  time  secure  uniform  circulation.  If 
circulation  is  excessive  in  any  place  it  simply  means  waste  of  energy 
but  no  injury  to  the  lumber. 

The  best  method  of  piling  is  one  which  permits  the  heated  air  to 
pass  through  the  pile  in  a  somewhat  downward  direction.  The  natu- 
ral tendency  of  the  cooled  air  to  descend  is  thus  taken  advantage  of 
in  assisting  the  circulation  in  the  kiln.  This  is  especially  important 
70253°—  Bull.  509—17 2 


10  BULLETIN   509,  U.  S.  DEPARTMENT   OF   AGRICULTURE. 

when  cold  or  green  lumber  is  first  introduced  into  the  kiln.  But 
even  when  the  lumber  has  become  warmed  the  cooling  due  to  the 
evaporation  increases  the  density  of  the  mixture  of  the  air  and  vapor. 
Table  3  shows  analytically  that  the  spontaneous  cooling  of  the  mix- 
ture produced  by  the  evaporation  alone  increases  its  density.  This 
fact  is  of  great  significance,  and  the  method  of  piling  lumber  in  the 
Forest  Service  kiln  takes  advantage  of  this  principle. 

THEORY  AND  DESCRIPTION  OF  THE  FOREST  SERVICE  KILN. 

The  humidities  and  temperatures  in  the  piles  of  lumber  are  largely 
dependent  upon  the  circulation  of  air  within  the  kiln.  The  tempera- 
ture and  humidity  within  the  kiln,  taken  alone,  are  no  criterion  of 
the  conditions  of  drying  within  the  pile  of  lumber  if  the  circulation 
in  any  portion  is  deficient.  It  is  possible  to  have  an  extremely  rapid 
circulation  of  the  air  within  the  dry  kiln  itself  and  yet  have  stag- 
nation within  the  pile,  the  air  passing  chiefly  through  open  spaces 
and  channels.  Wherever  stagnation  exists  or  the  movement  of  air 
is  too  sluggish  the  temperature  will  drop  and  humidity  increase, 
perhaps  to  the  point  of  saturation. 

When  in  large  kilns  the  forced  circulation  is  in  the  opposite  di- 
rection from  that  induced  by  the  cooling  of  the  air  by  the  lumber 
there  is  always  more  or  less  uncertainty  as  to  the  movement  of  the 
air  through  the  piles.  Even  with  the  boards  placed  edgewise,  with 
stickers  running  vertically,  and  with  the  heating  pipes  beneath  the 
lumber,  it  was  found  that  although  the  air  passed  upward  through 
most  of  the  spaces  it  was  actually  descending  through  others,  so  that 
very  unequal  drying  resulted.  While  edge  piling  would  at  first 
thought  seem  ideal  for  the  freest  circulation  in  an  ordinary  kiln  with 
steam  pipes  below,  it  in  fact  produces  an  indeterminate  condition; 
air  columns  may  pass  downward  through  some  channels  as  well  as 
upward  through  others,  and  probably  stagnate  in  others.  Neverthe- 
less, edge  piling  is  greatly  superior  to  flat  piling  where  the  heating 
system  is  below  the  lumber. 

From  experiments  and  from  a  study  of  conditions  in  commercial 
kilns  the  idea  was  developed  of  so  arranging  the  parts  of  the  kiln 
and  the  pile  of  lumber  that  advantage  might  be  taken  of  this  cooling 
of  the  air  to  assist  the  circulation.  That  this  can  be  readily  accom- 
plished without  doing  away  with  the  present  features  of  regulation 
of  humidity  by  means  of  a  spray  of  water  is  clear  from  figure  1, 
which  shows  a  cross  section  of  the  improved  humidity-regulated 
dry  kiln. 

In  the  form  shown  in  the  sketch  a  chamber  or  flue  B  runs  through 
the  center  near  the  bottom.  This  flue  is  only  about  6  or  7  feet  in 
height  and,  together  with  the  water  spray  F  and  the  baffle  plates  D  D, 
constitutes  the  humidity-control  feature  of  the  kiln.  This  control 
of  humidity  is  effected  by  the  temperature  of  the  water  used  in  the 


HUMIDITY-REGULATED   AXD   RECIRCULATING   DRY    KILN. 


11 


spray.  This  spray  completely  saturates  the  air  in  the  flue  B  at  what- 
ever predetermined  temperature  is  required.  The  baffle  plates  D  D 
are  to  separate  all  entrained  particles  of  water  from  the  air,  so  that 
it  is  delivered  to  the  heaters  in  a  saturated  condition  at  the  required 
temperature.  This  temperature  is,  therefore,  the  dew  point  of  the 
air  when  heated  above,  and  the  method  of  humidity  control  may 
therefore  be  called  the  dew-point  method.  It  is  a  very  simple  matter 
by  means  of  the  humidity  diagram,1  or  by  a  hydrodeik,  to  determine 
what  dew-point  temperature  is  needed  for  any  desired  humidity 
above  the  heaters. 


FIG.    1. — Diagrammatic   section   of   improved   dry  'kiln   with    spray   chambers    in    center. 

Double-truck  form. 

Besides  regulating  the  humidity  the  spray  F  also  acts  as  an  ejector 
and  forces  a  circulation  of  air  through  the  flue  B.  The  heating  sys- 
tem H  is  concentrated  near  the  outer  walls,  so  as  to  heat  the  rising 
column  of  air.  The  temperature  within  the  drying  chamber  is  con- 
trolled by  means  of  any  suitable  thermostat,  actuating  a  valve  on  the 
main  steam  line.  The  lumber  is  piled  in  such  a  way  that  the 
stickers  slope  downward  toward  the  center. 

M  is  an  auxiliary  steam  spray  pointing  downward  for  use  at  very 
high  temperatures.  C  is  a  gutter  to  catch  the  precipitation  and 

1  Forest  Service  Bulletin  104,  "  Principles  of  Drying  Lumber  at  Atmospheric  Pressure 
and  Humidity  Diagram,"  Superintendent  of  Documents,  Government  Printing  Office, 
Washington,  D.  C.  Price,  5  cents.  Lumber  World  Review,  Feb.  10.  1915. 


12 


BULLETIN   509,  U.  S.  DEPARTMENT   OF   AGRICULTURE. 


conduct  it  back  to  the  pump,  the  water  being  recirculated  through 
the  sprays.  G  is  a  pipe  condenser  for  use  toward  the  end  of  the 
drying  operation.  K  is  a  baffle  plate  for  diverting  the  heated  air  and 
at  the  same  time  shielding  the  under  layer  of  boards  from  direct 
radiation  of  the  steam  pipes. 

The  operation  of  the  kiln  is  simple.  The  heated  air  rises  above  the 
pipes  H  H  at  the  sides  of  the  piles  of  lumber.  As  it  comes  in  con- 
tact with  the  piles  portions  of  it  are  cooled  and  pass  downward  and 
inward  through  the  layers  of  boards  into  the  space  between  the  con- 


FIG.  2. — Diagrammatic  section  of  improved  dry  kiln  with  spray  chambers  on  sides.     Double- 
truck  form. 

densers  G  G.  Here  the  column  of  cooled  air  descends  into  the  spray 
flue  B,  where  its  velocity  is  increased  by  the  force  of  the  water  spray. 
It  then  passes  out  from  the  baffle  plates  to  the  heaters  and  repeats  the 
cycle. 

Various  modifications  of  this  arrangement  may  be  made.  For 
instance,  a  single-track  kiln  may  be  used.  This  form  would  be  repre- 
sented by  simply  dividing  the  diagram  vertically  into  two  parts  by 
extending  the  wall  G  (on  the  left  side)  upward  to  represent  the  outer 
wall,  and  erasing  the  part  to  the  left  of  this  line.  Or,  again,  the 
spray  chambers  may  be  kept  on  the  sides  as  shown  in  figure  2.  The 
lumber  would  then  slope  in  the  opposite  direction  with  respect  to 


HUMIDITY-REGULATED  AXD  RECIRCULATING   DRY   KILN.  13 

the  center  of  the  kiln  and  the  air  would  rise  in  the  center  and 
descend  on  the  sides. 

One  of  the  greatest  advantages  of  this  natural  circulation  method 
is  that  the  colder  the  lumber  when  placed  in  the  kiln  the  greater  is 
the  movement  produced,  under  the  very  conditions  which  call  for 
the  greatest  circulation — just  the  opposite  of  the  direct-circulation 
method.  This  is  a  feature  of  the  greatest  importance  in  winter,  when 
the  lumber  is  put  into  the  kiln  in  a  frozen  condition.  One  truck 
load  of  lumber  at  60  per  cent  moisture  may  easily  contain  over 
7,000  pounds  of  ice. 

In  the  matter  of  circulation  the  kiln  is,  in  fact,  self -regulatory — 
the  colder  the  lumber  the  greater  the  circulation  produced,  with  the 
effect  increased  toward  the  cooler  and  wetter  portions  of  the  pile. 

Preliminary  steaming  may  be  used  in  connection  with  this  kiln, 
but  experiments  indicate  that  ordinarily  it  is  not  desirable,  since 
the  high  humidity  which  can  be  secured  gives  as  good  results,  and, 
being  at  as  low  a  temperature  as  desired,  much  better  results  in  the 
case  of  certain  difficult  woods  like  oak,  eucalyptus,  etc. 

This  kiln  has  another  advantage  in  that  its  operation  is  entirely 
independent  of  outdoor  atmospheric  conditions,  except  that  baro- 
metric pressures  will  affect  it  slightly. 

THEORETICAL  DISCUSSION  OF  EVAPORATION. 

In  considering  the  drying  effect  of  vapor  alone  (superheated 
steam)  and  of  air  mixed  with  the  vapor,  one  very  significant  fact 
must  be  noticed.  Saturate  vapor  alone  in  cooling  and  in  order  to 
remain  saturate  must  absorb  heat.  Its  specific  heat  is  negative,  so 
that  the  only  way  it  can  heat  a  body  is  by  condensation.  It  is, 
therefore,  incapable  of  producing  evaporation.  When  air  is  present 
with  the  saturate  vapor,  however,  the  air  can  supply  some  of  this 
heat,  according  to  the  pressure  of  the  air  present,  so  there  will  be  less 
condensation. 

Still  more  important  is  the  fact  that  when  air  is  present  with  the 
vapor  sufficient  heat  can  be  supplied  to  the  body  being  dried  by  means 
of  the  air  without  greatly  superheating  the  vapor,  thus  keeping  a 
high  relative  humidity  and  at  the  same  time  supplying  a  sufficient 
amount  of  heat  to  carry  on  the  evaporation.  With  vapor  alone 
(superheated  steam)  a  relatively  high  degree  of  superheating,  which 
means  a  correspondingly  low  relative  humidity,  is  required  in  prac- 
tice in  order  to  supply  the  necessary  heat  for  evaporation,  after  the 
material  has  become  heated  through  to  the  temperature  of  the  sat- 
urated vapor  at  the  pressure  used.  Remember  that  the  temperature 
of  the  wet  wood  corresponds  to  that  of  the  wet  bulb  in  the  hygrom- 
eter when  air  is  present,  but  very  nearly  to  the  dew  point  in  the 
presence  of  superheated  vapor  alone. 


14  BULLETIN  50$,  U.  S.  DEPARTMENT  OF   AGRICULTURE. 

EVAPORATION  IN  THE  ABSENCE  OF  AIR. 

In  vapor  alone,  no  air  being  present,  evaporation  from  a  surface 
of  water  takes  place  at  the  dew  point,  but  when  the  water  is  inti- 
mately contained  in  other  substances  the  temperature  must  be  higher 
than  the  dew  point.  If  air  is  present  it  retards  the  rate  of  evapora- 
tion from  a  free  surface  of  water,  so  that  the  surface  is  warmer  than 
the  dew  point,  depending  upon  the  degree  of  relative  humidity  in 
the  air.  While  the  surface  of  wood  is  wet  its  temperature  will  not 
rise  above  that  of  the  wet  bulb  in  the  presence  of  air,  nor  above  the 
dew  point  in  superheated  vapor  alone.  As  it  becomes  drier,  how- 
ever, its  temperature  will  rise,  due  to  its  affinity  for  retaining  mois- 
ture. In  the  former  condition  there  is  no  danger  of  too  rapid  drying, 
but  in  the  latter  condition,  if  the  humidity  is  too  low  or  the  superheat 
too  high,  the  drying  from  the  surface  may  become  more  rapid  than 
the  rate  at  which  the  moisture  is  transmitted  from  the  center,  and 
casehardening  results. 

In  considering  the  manner  in  which  drying  takes  place  in  super- 
heated steam,  suppose  the  pressure  is  atmospheric  and  that  a  wet 
piece  of  wood  has  been  heated  in  saturated  steam  to  212°  F.  No 
evaporation  will  take  place  until  additional  heat  is  added.  Now, 
suppose  steam  superheated  to  232°  F.  or  20°  of  superheat  is  in- 
troduced. The  portion  immediately  in  contact  with  the  surface  of 
the  wet  wood  will  be  cooled  to  212°  F.,  and  in  so  doing  it  will 
vaporize  a  certain  portion  of  water  from  the  surface.  As  the  specific 
heat  of  this  steam  is,  in  round  terms,  one-half,  and  as  it  requires 
about  1,000  thermal  units  to  vaporize  one  unit  of  water,  to  vaporize 
a  single  molecule  of  water  at  212°  F.  will  require  contact  of  100  of 
the  molecules  of  superheated  steam  at  232°  F.  We  will  then  have  101 
molecules  of  steam  in  the  saturated  condition  at  212°  F.  Evapora- 
tion must  then  cease  unless  this  saturated  steam  is  replaced  by  some 
fresh  superheated  steam.  Evaporation  from  a  free  surface  of  water 
in  the  absence  of  air  (in  superheated  steam)  always  takes  place  at 
the  boiling  point  (which  in  this  case  is  the  same  as  the  dew  point). 
If,  however,  there  is  a  deficiency  of  water  in  the  wood  more  heat  will 
be  required  to  separate  it  and  to  vaporize  it,  and  evaporation  will 
take  place  at  a  higher  temperature  than  the  dew  point.  In  fact, 
evaporation  may  cease  altogether  in  the  superheated  steam,  and  a 
higher  degree  of  superheating  be  required  (which  is  equivalent  to  a 
lower  humidity)  to  get  the  moisture  out  of  the  wood.  In  the  case 
of  a  surface  of  free  water  the  rate  of  evaporation  depends  entirely 
upon  the  amount  of  heat  transmitted  to  the  water,  whether  by  in- 
creasing the  circulation  or  by  increasing  the  degrees  of  superheat. 
In  the  latter  case,  when  the  moisture  is  intimately  contained  in  the 


HUMIDITY-REGULATED  AND  RECIRCULATING  DRY   KILN.  15 

wood,  the  rate  depends  largely  upon  the  relative  humidity.1  There 
is  a  balance  between  what  might  be  termed  the  retentive  or  attractive 
property  of  the  wood,  "  hygroscopicity,"  and  the  tendency  of  the 
moisture  to  vaporize.  It  is  the  difference  between  the  tension  of  the 
vapor  at  the  higher  temperature  of  the  wood  and  the  tension  actually 
existing  in  the  space  surrounding  the  wood.  This  retentive  prop- 
erty increases  as  the  wood  becomes  drier  and  decreases  as  it  ap- 
proaches the  wet  condition.  Experiments  indicate  that  generally 
it  is  nearly  inversely  proportional  to  the  amount  of  moisture  remain- 
ing in  the  wood. 

EVAPORATION  WHEN  AIR  IS  PRESENT. 

When  air  is  present  with  the  superheated  steam  or  water  vapor 
the  conditions  are  quite  different.  Vaporization  of  a  particle  from 
the  surface  of  the  free  water  is  retarded  by  the  air  pressure,  so  that 
the  temperature  of  the  water  may  be  raised  above  the  dew  point.2 

The  air  now,  as  well  as  the  vapor,  conducts  heat  to  the  water,  so  that 
the  rate  of  evaporation  at  given  pressures  depends  not  alone  on  the 
quantity  of  heat  supplied  (by  circulation  and  degree  of  superheat- 
ing) but  upon  the  relative  amounts  of  vapor  and  air  present.  That 
is  to  say,  the  lower  the  relative  humidity  the  greater  is  the  rate  of 
evaporation  at  a  given  temperature  and  pressure.  The  temperature 
of  the  water  will  correspond  to  that  of  the  wet  bulb,  and  not  to  that 
of  the  dew  point.  When  the  wood  becomes  partially  dried  its  tem- 
perature will  rise,  as  in  the  case  of  superheated  steam,  and  it  may 
be  heated  even  above  the  boiling  point  at  the  given  pressure  without 
giving  up  all  of  its  moisture,  provided  there  is  some  vapor  in  the  air. 

CONCLUSIONS  AS  TO  DRYING  IN  VAPOR  ALONE  AND  IN  AIR  AND  VAPOR. 

Thus  it  is  seen  that  the  rate  of  drying  may  be  controlled  by  the 
relative  humidity,  provided  there  be  sufficient  circulation  to  supply 
the  heat  required.  In  the  case  of  steam  alone,  the  rate  of  drying,  as 
just  shown,  depends  upon  the  quantity  of  circulation  as  well  as  the 
degree  of  superheating.  Hence  the  conclusion  follows  that  moist 
air,  with  ample  circulation,  should  give  more  uniform  drying 
throughout  than  superheated  steam,  which  varies  with  the  rate  of 
circulation  in  each  portion. 

1  In  using  the  term  relative  humidity  as  applied  to  superheated  steam  it  is  understood 
to  mean  the  ratio  of  the  actual  vapor  pressure  to  that  of  the  pressure  of  saturated  vapor 
al  the  given  temperature,  as  explained  before. 

2  In  reality  what  probably  happens  is  that  the  layer  of  air  in  immediate  contact  with 
the  water  becomes  saturated  and  has  a  higher  vapor  pressure  corresponding  to  the  tem- 
perature of  the  surface  of  the  water,  and  the  air  retards  the  diffusion  of  this  vapor.     The 
temperature  of  the  water,  however,  can  not  exceed  the  boiling  point  for  the  given  pres- 
sure, at  which  point  the  conditions  must  become  the  same  as  those  for  superheated  steam 
alone,  since  then  the  air  will  become  entirely  displaced  by  the  water  vapor. 


16  BULLETIN  509,  U.  S.  DEPARTMENT  OF  AGRICULTURE. 

But  the  chief  difficulty  with  superheated  steam  at  or  above  atmos- 
pheric pressure  is  the  high  temperature  to  which  the  material  must 
be  subjected,  the  minimum  with  very  wet  wood  being  212°  F.,  and 
increasing  as  the  wood  dries.  Below  atmospheric  temperatures,  costly 
apparatus  is  required  for  operating  at  a  vacuum,  and  the  heating 
medium  is  attenuated,  requiring  an  excessive  volume  of  vapor  to  be 
circulated  if  the  danger  is  to  be  avoided  of  the  wood,  as  it  becomes 
dry  on  the  surface,  being  heated  too  high.  Instead  of  a  vacuum  the 
same  result  can  be  obtained  by  combining  air  with  the  vapor,  in 
which  case  the  air  makes  up  the  deficiency  of  pressure.  For  in- 
stance, a  vacuum  of  28  inches,  which  is  about  the  extreme  in  mechani- 
cal operations,  will  give  an  absolute  vapor  pressure  of  about  1  pound 
and  a  temperature  of  101°  F.  for  saturated  conditions.  Precisely 
the  same  value  for  the  vapor  occurs  if  saturated  air  at  101°  F.  and 
atmospheric  pressure  is  used  instead,  in  which  case  the  additional 
heating  capacity  of  the  air  present  is  also  available.  There  would 
then  be  in  a  cubic  foot  of  space  vapor  pressure  of  1  pound  (nearly) 
per  square  inch  and  13.7  pounds  of  dry  air  pressure.  This  amount 
of  vapor  would  weigh  0.0029  pound  and  the  air  1/15.2  or  0.0658 
pound  (15.2  being  the  volume  in  cubic  feet  of  1  pound  of  dry  air  at 
13.7  pounds  pressure  and  101°  F.  temperature). 

HEATING   CAPACITIES    OF   AIR   AND   VAPOR    IN   MIXTURE. 

The  heating  capacity  of  the  vapor  in  this  cubic  foot  of  space, 
in  falling  1  degree,  from  102°  F.  to  101°  F.,  is  .0029 X-421^. 00122 
B.  t.  u.,  as  before,  while  that  of  the  air  present  is  .0658X-237=.0156 
B.  t.  u.,  or  more  than  ten  times  that  of  the  vapor  present.  The  total 
heating  capacity  of  1  cubic  foot  of  the  mixture,  in  falling  1  degree,  from 
102°  F.  to  101°  F.,  is  then  the  sum  of  these  two,  viz,  .01682  B.  t.  u. 
The  latent  heat  of  evaporation  at  101°  F.  being  1044,  it  will  require 
the  heat  given  up  by  1044/.01682=r62.206  cubic  feet  of  the  mixed 
air  and  vapor  falling  1  degree,  from  102°  F.  to  saturation  at  101° 
F.,  to  evaporate  1  pound.  This  is  very  much  less  than  that  required 
for  vapor  alone,  which,  as  will  be  shown  farther  on,  is  829,433  cubic 
feet.  In  fact,  the  quantity  in  volume  is  less  than  that  of  dry  air 
alone  at  212°  F.  and  one  atmospheric  pressure  (69,000),  as  figured 
farther  on.  If  the  vapor  is  superheated,  say,  to  112°  F.,  its  pres- 
sure remaining  the  same  as  before,  this  is  simply  equivalent,  so  far 
as  the  vapor  is  concerned,  to  air  at  atmospheric  pressure  with  a  rela- 
tive humidity  of  less  than  saturation.  In  this  case  the  relative 
humidity  would  be  the  pressure  of  the  actual  vapor — 0.972  pound 
per  square  inch — divided  by  the  pressure  which  the  vapor  would 
have  if  it  were  saturated  at  112°  F.,  viz,  .972/1.34=73  per  cent 
humidity. 

1  The  specific  heat  of  superheated  vapor  at  this  temperature  is  0.421  as  given  by  Thiesen. 


HUMIDITY-REGULATED  AND  RECIRCULATING  DRY   KILN.  17 

It  should  now  be  evident  that  superheated  vapor  is  the  same  thing 
as  moist  air  with  the  air  removed.  The  same  effects  upon  the  mate- 
rial to  be  dried  are  produced  in  both  cases,  as  far  as  the  vapor  is 
concerned ;  but  in  the  case  of  moist  air,  the  effect  of  the  air  is  added 
to  that  of  the  vapor.  The  same  laws  apply  to  the  vapor,  whether  the 
air  is  present  or  absent.  The  air  conveys  heat,  but  by  its  presence 
retards  the  diffusion  of  the  vapor,  and  consequently  retards  the  rate 
of  evaporation. 

RELATIVE  HEATING  CAPACITIES  OF  AIR  AND  VAPOR. 

To  compare  the  relative  heating  capacities  of  dry  air  and  of  super- 
heated vapor,  the  following  deductions  are  made :  The  specific  heat  of 
water  vapor  at  a  pressure  of  one  atmosphere  is  0.475 ;  that  is  to  say, 
1  pound  of  superheated  steam  in  falling  1°  F.  gives  up  0.475  British 
thermal  unit.  To  evaporate  1  pound  of  water  at  212°  F.,  therefore, 
will  require  the  heat  given  up  by  966  (latent  heat  at  212°  F.)-r- 
.475=2034  pounds  of  steam  falling  1  degree.  At  212°  F.  the  volume 
per  pound  is  26.78  cubic  feet;  therefore,  2034X26.78=54,470  cubic 
feet  of  superheated  steam  falling  1  degree  are  required  to  evaporate 
1  pound  of  water.  The  specific  heat  of  dry  air  is  0.237  and  the  vol- 
ume of  1  pound  is  16.93  (0.05907  pound  per  cubic  foot)  at  212°  F. 
and  atmospheric  pressure.  Therefore,  to  evaporate  1  pound  of  water 
at  212°  F.  (966  B.  t,  u.)  will  require  the  heat  given  up  by 

16  93 

966X  -057-  —69,000  cubic  feet  of  dry  air  falling  1  degree.    Thus  it 
.Zo  i 

is  seen  that  the  heating  capacity  per  unit  of  volume  of  superheated 
steam  at  atmospheric  pressure  is  but  little  greater  than  that  of  dry 
air  at  the  same  temperature  and  pressure,  in  the  ratio  of  69,000  to 
54.470,  or  about  5  to  4.  At  temperatures  above  212°  F.  and  the  same 
pressure  of  one  atmosphere  a  greater  volume  is  necessary  to  produce 
the  same  effect,  since  the  gas  and  vapor  expand  with  temperature, 
but  the  ratio  of  the  heating  capacity  of  superheated  steam  and  dry  air 
remains  very  nearly  the  same.  The  specific  heat  of  vapor  increases 
slightly  at  higher  temperatures.  Thus,  figuring  in  a  similar  man- 
ner, it  will  be  found  that  at  five  atmospheres  pressure  (59  pounds 
gauge)  the  heating  ratio  of  equal  volumes  of  steam  and  air  is  1.42 
to  1,  and  at  1  pound  absolute  pressure  or  a  vacuum  of  28  inches,  it 
is  1.104  to  1.  The  volume  of  steam  at  five  atmospheres  pressure  and 
306°  F.  in  falling  1  degree  necessary  to  evaporate  1  pound  of  water 
at  this  pressure  and  temperature  is  10,336  cubic  feet,  and  at  a 
vacuum  of  28  inches  at  101°  F.  it  is  829,433  cubic  feet. 

Thus  it  is  seen  that  there  is  but  little  advantage,  from  the  point 
of  view  of  the  volume  of  gas  to  be  moved,  in  the  use  of  superheated 
steam  over  that  of  dry  air. 


18  BULLETIN   509,  U.  S.  DEPARTMENT   OF   AGRICULTURE. 

In  this  discussion  a  cubic  foot  of  space  has  been  used  as  the  basis 
of  the  calculations.  In  analyzing  the  heat  quantities  in  the  drying 
operation  it  will  be  easier  to  use  1  pound  of  dry  air  as  a  basis,  with 
its  accompanying  moisture,  and  follow  it  through  its  various  stages. 
Its  volume  will  therefore  not  remain  fixed,  but  will  change  with 
every  change  in  temperature,  and  consequently  the  degree  of  satura- 
tion produced  by  a  definite  amount  of  moisture  accompanying  it  will 
depend  upon  the  volume  which  it  occupies. 

THEORETICAL  ANALYSIS  OF  HEAT  QUANTITIES. 

For  this  purpose*  the  simplest  way  will  be  to  follow  a  pound  of 
dry  air  through  a  drying  cycle  as  a  basis  for  computations.  While 
in  reality  the  vapor  does  not  enter  the  air  like  water  in  a  sponge,  but 
occupies  the  same  space  whether  air  is  present  or  not,  we  may,  for 
convenience,  conceive  of  a  pound  of  air  as  containing  a  certain  amount 
of  vapor,  which,  in  reality,  means  that  the  space  occupied  by  a 
pound  of  dry  air  under  given  conditions  contains  a  certain  amount  of 
vapor. 

VAPOR  AND  AIR  IN  MIXTURE. 

As  already  explained,  the  total  pressure  always  is  the  sum  of  the 
individual  pressures  of  the  air  alone  plus  the  vapor  alone.  Thus 
we  may  speak  of  a  pound  of  air  as  being  wholly  or  partially  satu- 
rated with  vapor,  meaning  that  it  is  the  space  occupied  by  the  pound 
of  air  which  is  in  this  condition  of  vapor.  If  a  pound  of  air  said  in 
this  sense  to  contain  a  given  weight  of  vapor  is  heated  a  given  amount 
under  a  pressure  of  one  atmosphere,  both  air  and  vapor  will  expand 
the  same  amount,  so  that  at  the  new  temperature  both  will  occupy 
the  same  relative  amount  of  space;  the  pound  of  air,  however,  will 
still  contain  the  same  weight  of  vapor.  The  amount  of  vapor  con- 
tained in  a  pound  of  air  alone,  when  it  is  saturated,  can  not  be  used 
as  the  divisor  in  obtaining  the  relative  humidity  when  compared 
to  the  amount  of  vapor  actually  contained  in  the  pound  of  air  alone, 
because  when  the  air  is  saturated  the  pressure  of  the  air  alone  will 
have  been  reduced,  corresponding  to  the  increase  in  the  vapor  pres- 
sure (since  the  sum  of  the  two  make  up  one  atmosphere),  so  that  for 
a  pound  of  air  a  much  greater  space  is  required,  and,  consequently,  an 
equivalently  greater  weight  of  vapor  to  occupy  this  larger  space. 
For  relative  humidity  it  is  necessary  to  compare  the  weights  of  vapor 
which  occupy  the  same  amount  of  space  when  partially  or  wholly 
saturated,  or,  better  still,  to  compare  the  vapor  pressures. 

CYCLE  IN  DRYING  OPERATION   OF   1  POUND   OF  AIR. 

In  following  the  pound  of  dry  air  through  its  cycle  of  opera- 
tion, let  the  air  enter  the  heater  either  from  outside  or  from  the 
spray  chamber  at  temperature  t15  and  let  it  contain  d±  pounds  of 


HUMIDITY-REGULATED  AXD  RECIRCULATING  DRY   KILN. 


19 


vapor.  (See  fig.  3.)  After  passing  through  the  heater  both  the  air 
and  the  vapor  are  raised  to  the  temperature  t2.  Each  pound  of  air 
still  contains  d±  pounds  of  moisture,  since  the  vapor  expands  to  the 
same  extent  as  the  air  if  no  vapor  is  added  or  subtracted  during  the 
heating  from  tx  to  t2.  In  passing  through  the  lumber,  the  air  and 
vapor  become  cooled  to  t3,  and  an  amount  of  moistnre,  w,  is  added 
from  the  evaporation,  so  that  the  pound  of  air  at  temperatures 
t3  now  contains  d3=  (c^+w)  pounds  of  moisture.  Thence  they  either 
escape  into  the  outer  air,  as  in  a  ventilating  kiln,  or  pass  into  the 
spray  chamber,  where  the 
heat  added  by  the  heater 
and  the  extra  amount  of 
moisture  w  is  removed  from 
the  pound  of  air  into  the 
spray  water,  and  is  re- 
turned at  the  initial  tem- 
perature tx  saturated  to  re- 
peat the  cycle.  The  changes 
in  total  pressure  will  be  so 
slight  that  they  may  be  neg- 
lected, and  the  whole  op- 
eration considered  to  take 
place  at  a  uniform  pressure 
of  one  atmosphere.  Let  r 
equal  the  specific  heat  of  air 
at  constant  pressure,  and  s 
that  of  superheated  vapor. 
These  will  be  taken  as  0.237 
and  0.475,  respectively. 
Then  the  quantity  of  heat 
imparted  to  the  pound 
of  air  and  its  accompany- 
ing dx  pounds  of  vapor  by  the  heater  is  (1),  (.237+d1X-475)  (t2— tj 
and  the  amount  of  heat  given  up  in  evaporating  the  water  w  is 
(2),  (.237+d1X.47o)  (t2— 13).  The  amount  of  water  evaporated 
is  w=(d3— di).  Now  the  heat  required  to  evaporate  the  water  w 
in  continual  operation  will  be  that  required  to  raise  it  from  its  initial 
temperature  to  the  evaporating  point,  plus  the  latent  heat  of  vapori- 
zation at  this  point ;  also  the  heat  necessary  to  raise  the  temperature 
of  the  wood  alone  the  same  amount.  As  the  latter  is  small,  it  will 
be  neglected.  Suppose  that  the  initial  temperature  of  the  outside  air 
and  of  the  wet  wood  is  32°  F.  Then  the  heat  required  is  simply  the 
total  heat  H  of  w  pounds  of  vapor  at  the  temperature  t3  (nearly).1 


TD 


22 


FIG.  3. — Diagrammatic  plan  of  drying  cycle. 


1  Evaporation  will  actually  take  place  at  the  temperature  of  the  wet  bulb  if  the  air  Is 
not  saturated,  after  which  the  vapor  is  superheated  to  ta. 


20  BULLETIN  509,  U.  S.  DEPARTMENT  OF   AGRICULTURE. 

Hence  (3),   (.237+d±  .475)    (t2-t3)=wH=(d.-d1)  H  or  -^"4  = 
In  this  equation  t2  is  a  known  quantity,  being  de- 


—  . 
!  .475 

pendent  upon  the  kind  and  condition  of  the  material  being  dried. 
d-L  is  known,  being  the  weight  of  moisture  of  the  outside  air  per 
pound  of  dry  air,  or  the  weight  required  to  saturate  1  pound  of  air 
in  the  spray  kiln  at  the  temperature  t^  H  is  known  approximately 
(but  not  exactly,  since  its  value  varies  with  t3,  or  more  properly  with 
the  wet-bulb  temperature),  and  may  at  first  be  assumed  for  some 
temperature  between  t2  and  t15  and  afterwards  be  correctly  assigned. 
t3  and  d3  are  the  unknown  quantities  required.  If  the  air  is  to  be 
considered  saturated  at  t3,  then  ts  and  ds  are  dependent  variables, 
their  equation  being  that  of  the  curve  of  saturation  for  water  vapor. 
As  the  equation  is  complex,  their  relative  values  can  be  more  readily 
obtained  from  a  table  of  saturated  vapor,  and  successive  values  sub- 
stituted in  equation  (3)  until  the  equation  is  fulfilled.  Having  thus 
determined  ts  approximately,  the  correct  value  for  H  may  be  in- 
serted and  the  more  exact  value  of  t3  determined.  This  has  been 
done  by  E.  Hausbrand  in  "  Drying  by  Means  of  Air  and  Steam  "  1 
for  diffrent  temperatures  of  tj.  and  t2,  as  well  as  for  different  humidi- 
ties and  pressures. 

EFFICIENCY  OF  OPERATION. 

With  no  air  present  —  that  is  to  say,  with  water  vapor  alone  under 
a  so-called  "  vacuum,"  or  with  "  superheated  steam'"  at  pressures  of 
one  atmosphere  or  greater  —  all  the  heat  may  be  utilized  in  evaporating 
the  moisture,  the  leaving  and  entering  temperatures  being  the  same 
and  the  pressure  constant.  With  air  present,  however,  and  the  pres- 
sure constant,  it  follows  that  if  the  entering  air  is  saturated  the 
leaving  air  must  be  at  a  higher  temperature,  in  order  that  it  may 
contain  the  additional  vapor  at  the  same  pressure.  Thus  in  raising 
the  temperature  of  the  air  leaving  the  lumber  a  greater  amount  of 
heat  is  required  than  that  utilized  in  evaporation. 

There  is  another  combination  of  conditions  possible  in  which  the 
temperature  at  exit  may  be  the  same  or  even  less  than  that  of  the 
entering  air  or  vapor.  With  air  present  this  is  only  possible  by  de- 
creasing the  pressure  below  that  of  the  entering  saturated  air.  In 
this  case  the  heat  supplied  may  be  even  less  than  the  theoretical 
amount  required  for  vaporization,  and  the  theoretical  efficiency  as 
reckoned  by  temperatures  is  more  than  100  per  cent.  The  advantage 
gained  here  is  at  the  expense  of  the  heat  energy  in  the  departing  air 
and  vapor,  being  somewhat  analogous  to  the  case  of  the  condenser 
in  a  steam  engine.  The  gain  in  heat  is  from  the  fact  that  the  enter- 

1  Translation  from  the  German  by  Wright.     Published  by  Scott  Greenwood  &  Sons,  1901. 


HUMIDITY-REGULATED  AND  RECIRCULATING   DRY   KILN.  21 

ing  air  is  at  a  higher  temperature  than  the  leaving  air.  If  the  en- 
tering air  is  not  saturated,  a  similar  condition  is  possible,  since  some 
evaporation  may  take  place  without  necessitating  a  higher  tempera- 
ture of  the  leaving  air. 

From  the  foregoing  it  might  be  concluded  that  the  use  of  a  vacuum 
or  of  superheated  steam  would  be  the  most  economical  way  in  which 
to  dry  materials.  In  practice,  however,  the  vacuum  has  certain  dis- 
advantages, as  explained  heretofore,  the  chief  one  being  the  greater 
volume  of  vapor  required  and  the  difficulty  of  producing  a  uniform 
circulation  of  vapor  at  high  attenuation.  The  other  drawback  is  the 
expense  of  the  apparatus  and  difficulty  of  operation  at  pressures 
other  than  atmospheric.  With  superheated  steam  the  temperature 
is  too  high  for  most  woods. 

CONCRETE   EXAMPLES    OF   RELATIONS   OF   HEAT   QUANTITIES. 

To  illustrate  the  relations  of  these  quantities  under  the  various 
conditions,  let  us  take  a  concrete  example  where  the  initial  tempera- 
ture of  the  air  is  32°  F.  and  the  air  is  saturated  both  at  the  entrance 
and  upon  leaving.  This  is  heated  to  158°  F.  and  then  passed  through 
the  material  to  be  dried.  The  volume  of  the  gas  required  at  the  tem- 
perature of  158°  F.  and  the  theoretically  least  possible  expenditure 
of  heat  required  to  evaporate  1  pound  of  water  from  an  initial  tem- 
perature of  59°  F.  at  various  pressures  are  given  below  in  Table  1. 

TABLE  1. — Volume  of  gas  required  at  a  temperature  of  158°  F.  and  the  theoreti- 
cally least  possible  expenditure  of  heat  required  to  evaporate  1  pound  of  water 
from  an  initial  temperature  of  59°  F.  at  various  pressures. 


Absolute  pressures. 

Volume. 

Total  heat 
required. 

1  \  atmospheres  

Cubicfeet. 
695 

B.  t.  u. 
2  010 

llatmospliere  —  760mm  of  mercury 

876 

1  692 

500  mm  of  mercury  partial  vacuum 

1  247 

1  578 

250  mm.  of  mercury,  partial  vacuum  .  .  . 

2  121 

l'346 

Using  steam  alone  superheated  from  140°  to  158°  F.  at  pressure  of  148  mm.  of  mer- 
cury, corresponding  to  saturated  conditions  at  140°  F  

16  821 

1  125 

The  minimum  theoretical  expenditure  of  heat,  as  here  calculated, 
has  no  direct  bearing  on  the  efficiency  of  any  method  of  drying 
lumber,  since  the  physical  requirements  of  the  lumber  may,  and 
generally  do,  demand  conditions  totally  incompatible  with  the  high- 
est theoretical  heat  efficiency.  They  apply  directly  only  to  the  evapo- 
ration of  a  free  body  of  water,  irrespective  of  length  of  time  re- 
quired and  with  no  radiation  losses.  The  calculations  are  useful, 
however,  in  showing  the  limiting  values  of  the  efficiency  which  it 
is  possible  to  attain  under  the  conditions  which  have  otherwise 
been  found  most  suitable  for  drying  the  lumber  in  question. 

It  is  instructive  to  know  the  highest  possible  theoretical  efficiency 
in  evaporating  a  pound  of  water  under  given  conditions,  considering 
no  losses  by  radiation  or  otherwise.  For  this  purpose  Table  2  has 


22 


BULLETIN   509,  U.  S.  DEPARTMENT'  OF   AGRICULTURE. 


been  worked  out,  assuming  the  water  to  start  with  an  initial  tem- 
perature of  59°  F.  and  to  evaporate  at  the  temperature  t3,  which  is 
the  temperature  of  the  leaving  air.  The  efficiency  here  expressed  is 
the  ratio  of  the  total  heat  of  water  vapor  at  t3  above  59°  F.  divided 
by  the  least  possible  expenditure  of  heat  necessary  to  evaporate  it 
under  the  assumed  conditions  of  the  entering  and  leaving  air  at 
atmospheric  pressure.  When  the  temperature  t±  of  the  entering  air 
approaches  that  of  the  heated  air  t2 — that  is,  when  a  high  humidity 
is  used — the  calculations  become  very  uncertain,  since  the  quantity  of 
air  called  for  under  the  assumed  conditions  approaches  infinity, 
while  the  temperature  differences  beUveen  t±  and  ts  become  infini- 
tesimal. 

The  minimum  volume  of  air  required  to  evaporate  1  pound  of 
water  is  also  given  in  Table  2. 

TABLE  2. — Maximum  possible  theoretical  heat  efficiency  of  evaporation  under 
given  conditions  (ti,  t2,  hi,  h3)  at  atmospheric  pressure  (760  mm.). 


Heat  con- 

Total 

sumed  to 

heat  of 

Entering  air. 

After  heating. 

Leaving  air. 

evaporate 
1  pound 

1  pound 
of  vapor 

Minimum 

of  water 

att3 

volume 

Efficiency 

from 

above 

of  air 

H-s-G. 

initial 

initial 

required. 

tempera- 

tempera- 

ture of 

ture  of 

ti 

hi 

ta 

h2 

ts 

h3 

59°  F. 

59°  F. 

A 

B 

c 

D 

E 

F 

G  . 

H 

J 

K 

0  F. 

Perct. 

o   F 

Perct. 

0  F. 

Perct. 

B.t.u. 

B.t.u. 

Cubic  ft. 

32 

100 

95 

11 

65 

75 

2,353 

1,074 

2,163 

0.457 

59 

100 

95 

31 

76 

75 

2,100 

1,078 

3,426 

.514 

32 

100 

158 

2 

84 

75 

,911 

1,080 

993 

.565 

59 

100 

158 

6 

92 

75 

,715 

1,082 

1,126 

.631 

86 

100 

158 

13 

107 

75 

,556 

1,087 

1,402 

.698 

32 

100 

212 

0+ 

97 

•75 

,758 

1,084 

694 

.617 

59 

100 

212 

2 

103 

75 

,572 

1,086 

731 

.690 

86 

100 

212 

4 

114 

75 

,422 

1,089 

796 

.767 

32 

100 

95 

11 

84 

25 

6,136 

1,080 

5,738 

.176 

32 

100 

158 

2 

110 

25 

2,972 

1,088 

1,495 

.366 

86 

100 

158 

13 

141 

25 

4,869 

1,098 

4,385 

.225 

32 

100 

212 

0+ 

126 

25 

2,352 

1,093 

930 

.457 

86 

100 

212 

4 

146 

25 

2,166 

1,099 

1,206 

.507 

32 

100 

95 

11 

60 

100 

1,974 

1,073 

1,836 

.544 

59 

100 

95 

31 

70 

100 

1,679 

1,076 

2,733 

.641 

86 

100 

95 

74 

88 

100 

1,476 

1,081 

9,725 

.733 

32 

100 

158 

2 

79 

100 

1,692 

1,079 

876 

.636 

86 

100 

158 

13 

99.5 

100 

1,390 

1,085 

1,329 

.781 

140 

100 

158 

63 

140.9 

100 

1,119 

1,098 

3,879 

.981 

32 

100 

212 

0+ 

90 

100 

1,582 

1,082 

625 

.684 

86 

100 

212 

4 

106 

100 

1,350 

1,087 

721 

.804 

176 

100 

212 

47 

176.5 

100 

1,130 

1,108 

2,002 

.972 

IN  WATER  VAPOR  ALONE. 


140 

100 

158 

63 

140 

100 

1,097 

1,097 

16,  418 

1.00 

212 

100 

230 

71 

212 

100 

1,119 

1,119 

3,657 

1.00 

212 

100 

320 

16 

212 

100 

1,121 

1,121 

664 

1.00 

HUMIDITY-EEGULATED  AXD  RECIRCULATING  DRY   KILN.  23 

GENERALIZATION. 

A  study  of  the  theoretical  heat  relations,  as  shown  by  Hausbrand's 
tables,  makes  possible  the  following  generalizations: 

1.  With  t2  constant  and  entering  air  saturated,  the  expenditure  of 
heat  is  less,  the  higher  the  temperature,  t±,  of  the  entering  air. 

2.  With  tx  constant,  the  expenditure  of  heat  is  less,  the  higher  the 
temperature,  t2,  to  which  the  air  is  heated. 

3.  Other  things  being  the  same,  the  heat  expenditure  increases 
rapidly  with  reduction  in  humidity  of  the  emergent  air. 

4.  Other  things  being  the  same,  the  heat  expenditure  is  less,  the 
lower  the  humidity  of  the  entering  air. 

5.  Other  things  being  the  same,  the  expenditure  of  heat  increases 
with  increase  of  pressure. 

6.  With  water  vapor  in  the  absence  of  air,  the  theoretical  efficiency 
becomes  100  per  cent. 

In  regard  to  the  weights  and  volumes  of  air  required,  the  following 
observations  are  obtained,  with  entering  air  saturated : 

With  t2  constant,  both  the  weights  and  volumes  of  air  required  to 
evaporate  1  pound  of  water  increases  with  increase  of  the  initial 
temperature,  t15  of  the  entering  air. 

With  tt  constant,  both  weighty  and  volumes  decrease  with  increased 
temperature,  t2,  of  the  heated  air. 

With  the  emergent  air  only  partially  saturated,  the  weights  and 
volumes  increase  with*  decrease  of  relative  humidity  in  the  emergent 
air. 

CONCLUSIONS  AS  TO   EFFICIENCY  OF  OPERATION. 

From  this  analysis  of  the  heat  equations  the  following  conclusions 
as  regards  the  efficiency  of  the  drying  may  be  drawn : 1 

1.  The  air  should  be  heated  to  the  highest  temperature  compatible 
with  the  nature  of  the  material  to  be  dried. 

2.  The  air  upon  leaving  the  apparatus  should  be  as  near  saturation 
as  practicable. 

3.  The  temperature  of  the  entering  air  should   be   as  high    as 
possible. 

APPLICATION   OF  ANALYSIS  TO  THE  WATER   SPRAY  OR  CONDENSING  KILNS. 

The  above  deductions  apply  to  any  form  of  moist-air  kiln.  The  fol- 
lowing have  more  especially  to  do  with  the  Forest  Service  water 
spray  humidity  regulated  kiln. 

The  amount  of  heat  absorbed  by  the  spray  water  and  the  con- 
densed moisture  aside  from  losses  through  the  kiln  walls  is  the 

1  It  should  be  noted,  as  stated  above,  that  these  deductions  apply  solely  to  the  evapo- 
rating process  alone,  from  a  theoretical  standpoint,  and  do  not  take  into  consideration 
heat  losses  through  the  kiln  walls  or  through  extraneous  conditions  ;  nor  do  they  signify 
•what  is  the  condition  best  suited  for  conducting  the  drying  operation  from  the  stand- 
point of  the  physical  effect  upon  the  wood. 


24  BULLETIN   509,  U.  S.  DEPARTMENT    OF   AGRICULTURE. 

difference  between  the  total  heat  in  the  saturated  air  as  it  leaves 
the  lumber  at  t3  and  the  total  heat  in  the  air  at  tx.  It  is,  in  fact> 
the  amount  of  heat  given  up  by  the  coils,  since  the  air  is  brought 
back  to  its  initial  state  in  the  cycle  and  the  water  evaporated 
from  the  wood  is  added  to  the  spray  water.  Hence  the  amount  of 
heat  removed  in  water  at  a  temperature  tx  is  (4) ,  G(t2—  tj  X  (c+sdx) , 
when  G  is  the  weight  of  dry  air  in  the  mixture  required  to  evaporate 
1  pound  of  water,  c  and  s  are  the  specific  heats  of  the  air  and  vapor. 
Of  this  the  amount  G(t3— tj  (c+sdj  represents  the  loss  not  ac- 
counted for  in  the  latent  heat  of  the  pound  of  water  which  has  been 
evaporated  and  is  taken  up  by  the  spray  water.  The  maximum 

possible  thermal  efficiency  is  therefore  (5),   f.2~,^  if  just  enough 

air  is  circulating  to  give  up  all  its  available  heat  to  the  evaporation 
of  the  water  so  that  it  leaves  the  lumber  in  a  saturated  condition. 
From  equation  (2)  and  (3)  the  value  of  t3  is  determined  for  any  given 
values  of  t±  and  t2.  These  values  may  be  most  readily  obtained  from 
the  tables  given  by  Hausbrand,  before  referred  to.  ti  and  t2  are 
arbitrary  values  determined  entirely  by  the  physical  conditions  of 
the  material  to  be  dried. 

In  actual  operation,  however,  the  efficiency  will  be  much  leSvS  than 
this  maximum,  since  the  air  leaving  will  not  be  saturated,  and  a 
much  larger  quantity  of  air  will  need  to  pass  through  the  material 
than  the  'minimum  indicated  by  the  equation.  If  no  evaporation 
takes  place,  all  the  heat  will  be  used  in  heating  and  cooling  the  cir- 
culating medium.  The  total  heat  used  per  pound  of  air  will  then 
be  (t2— tx)  (c+sd-J,  and  this  will  go  simply  to  heating  the  spray 
water. 

COMPARISON  OF  EFFICIENCY. 

Comparing  the  theoretical  efficiency  of  the  condensing  with  that  of 
the  ventilating  type  of  kiln,  it  will  be  seen  that  under  identical  run- 
ning conditions  its  efficiency  is  much  greater,  because  the  initial  tem- 
perature t±  is  very  much  higher.  Let  the  temperature  of  the  outside 
air  be  32°  F.,  so  that  the  water  has  to  be  raised  from  32°  F.  to  the  tem- 
perature of  evaporation  an  dthen  evaporated.  Let  the  air  leaving  the 
lumber  be  three-fourths  saturated,  75  per  cent  humidity.  Also  let 
t1=113°  and  t2=140°,  giving  a  relative  humidity  of  48  per 
cent.  Then  d±  for  1  pound  of  saturated  air  at  113  is  0.0653 
pound.  Substituting  those  values  in  equation  (3)  it  is  found  that 
t3=125°  and  d3=0.06889.  Since  w=d3— d15  the  number  of  pounds 

1  1 

of  air  required  to  evaporate  1  pound  of  water  is  G=  ~=  jriig~=*"^ 

which  contains  279X0.0653=18.2  pounds  of  vapor.  The  pressure 
of  the  saturated  vapor  alone  at  113°  is  71.4  mm.  of  mercury;  hence 
that  of  the  air  alone  is  760—71.4=688.6  mm.  of  mercury.  The 


HUMIDITY-REGULATED  AND  RECIRCULATING  DRY   KILN.  25 

volume  occupied  by  1  pound  of  dry  air  at  113°  and  a  pressure  of 
688.6  mm.  of  mercury  is  16  cubic  feet  (more  exactly  15.921),  which 
must  be  the  same  as  that  occupied  by  the  0.0654  pound  of  vapor 
present  in  the  pound  of  air.  As  279  pounds  of  air  are  required 
with  its  inherent  18.2  pounds  of  vapor,  the  volume  of  air,  or  com- 
bined air  and  vapor,  is  15.921X279=4,442  cubic  feet  at  113°.  At 
125°  this  will  occupy  4,535  cubic  feet. 

The  total  heat  consumed  is  279  (0.237+0.0653X0.475)  X  (140-113) 
=2,019  B.  t.  u.,1  of  which  the  useful  work  has  been  the  total  latent 
heat  of  1  pound  of  vapor  above  32°  F.  evaporated  at  116°  F.  (the 
wet-bulb  temperature)  and  superheated  to  125°  F.=  1,122  B.  t.  u. 
This  should  be  the  same  as  the  heat  given  out  by  the  air  and 
superheated  vapor  in  cooling  from  140°  F.  to  125°  F.,  279  (0.237+ 

0.0653X0.475)  X  (140—125)  =1,122.    The  thermal  efficiency  is  *^*= 

to  -  ^1 

=55'6  per  cent  Also     =55-6  per  cent 


Compare  this  first  with  a  ventilating  kiln  in  which  the  air  enters 
saturated  at  32°  F.,  is  heated  to  140°  F.,  and  leaves  at  75  per  cent  hu- 
midity, escaping  to  the  outer  air.  We  then  have 

^=32°,  d!=.00387  pound  per  pound  of  air 

t,=140° 

t3=calculated=80.2,  and  d3  at  75  per  cent  humidity=.01692. 
The  quantity  of  air  required  to  evaporate  1  pound  of  water  is  : 

G=.01692-.00387  =  76'6  P°Unds' 

This  air  contains  76.6X-00387=0.296  pound  of  vapor.     The  total  heat 
consumed  is: 

76.6  (.237+.00387X-475)    (140-32)  =1,969  B.  t.  u. 

The  thermal  efficiency  is         ~      =55.6  per  cent,  which  happens  to  be 

14(J  —  o£ 

the  same  as  in  the  condensing  kiln,  but  examination  will  show  at  once 
that  the  two  cases  are  not  analogous.     In  the  condensing  kiln  the 

1  Another  way  of  arriving  at  this  result  is  to  compare  the  total  heats  ;  thus,  in  the 
vapor  at  125°  and  75  per  cent  saturation  : 
Total  heat  in  the  air  alone  at  125°=279X  0.237  (125  —  32)  equals  ______________     6,  149 

Total  heat  in  saturate  vapor  at  the  dew  point  of  115°   (75  per  cent  humidity  at 

125°)  =279X0.06889  XI  117  equals  _______________________________________  21,  491 

Superheating  this  vapor   from  its   dew   point   of   115°   to   125°=279  X  0.06889  X 

0.475X10  equals  _______________________________________________________  91 


Total  at  125° 27,731 

At  the  initial  stage,  113°  : 

Total  heat  in  air=279  X  0.237  (113  —  32)  equals 5,356 

Total  heat  in  saturate  vapor  at  1 13°=279X  0.0653  X  1116.4  equals 20,  339 

Total  heat  at  113° 25,695 

The  difference,  27,731-25,695=2,036  B.  t.  u.,  is  the  heat  added  to  the  air.  This 
should  be  the  same  as  before,  namely,  2,019,  the  difference  being  in  inaccuracy  of  the 
constants  used. 


26  BULLETIN   509,  U.  S.  DEPARTMENT   OF   AGRICULTURE. 

humidity  after  heating  to  140°  F.  was  48  per  cent ;  in  the  other  kiln 
it  is  only  3  per  cent,  an  extremely  low  amount. 

For  a  correct  comparison,  the  condition  of  the  air  entering  the 
lumber  should  be  the  same  in  both  cases,  namely,  it  is  necessary  to 
raise  the  humidity  in  the  ventilating  kiln  from  3  per  cent  to  48  per 
cent.  This  can  be  done  by  allowing  live  steam  to  escape  into  the 
heated  air  sufficient  to  saturate  it  at  113°  F.,  the  dew  point  for  48 
per  cent  humidity.  Now,  if  1  pound  of  dry  air  saturated  at  32°  F. 
is  heated  to  113°  F.  it  will  still  contain  its  original  weight  of  vapor, 
namely,  0.00387  pound;  but  to  saturate  a  pound  of  air  at  113°  F.  re- 
quires 0.0653  pound  of  vapor;  consequently,  the  difference  between 
this  and  0.00387  or  0.06143  pound  of  vapor  must  be  added  for  each 
pound  of  air  at  113°  F.,  in  order  to  make  the  two  cases  comparable ; 
they  are  then  exactly  alike,  and  we  shall  have  for  our  kiln,  to  re- 
capitulate, as  before — 

^=113°  saturated 
t2=140°  humidity  48  per  cent 
t3=125°  humidity  75  per  cent. 

Number  of  pounds  of  air  required  to  evaporate  1  pound  of  water 
at  115°  from  initial  temperature  of  32°  =279— 

Total  heat  required=2,019  B.  t.  u. 
Heat  lost  *  2,019— 1,122=897  B.  t.  u. 

In  the  ventilating  kiln,  on  the  other  hand,  we  shall  have  by  com- 
parison : 

^=32°  saturated. 
t2 =140°  at  3  per  cent  humidity. 
t3=125°  humidity  75  per  cent. 
h2=heat  in  vapor  added  to  raise  the  humidity 
to  saturation  at  113°  F. ;  0.0614  pound  are  required  per  pound  of 
air.     The  total  heat  in  saturate  vapor   at  113°    above   32° =1,117 
B.  t.  u.  per  pound;  1,117X.0614=68.58  B.  t.  u.  required  per  pound 
of  air.     There  are  279  pounds  of  dry  air  required  as  in  the  other 
case.    68.5X279=19,134  B.  t.  u.,  which  must  be  added  as  vapor. 

K2=heat  required  to  raise  temperature  of  the  air  and  vapor  from 
32°  to  113°=279  (.237+.00387X.475)  (113-32°)  =5,396  B.  t.  u. 

Therefore,  in  this  case  the  total  heat  which  must  be  given  to  the 
air  to  evaporate  1  pound  of  water  is — 

B.  t.  u. 

Heat  given  by  coils  to  raise  the  air  from  32°  to  113°  equals 5,  39G 

Heat  given  by  coils  to  raise  saturate  air  from  113°  to  140°  as  before 

equals 2,  019 

Heat  supplied  in  vapor  equals 19, 134 

Total  heat  required 26,  549 

Heat  lost  (provided  it  all  escaped  to  the  air)  26,549  minus  1,122  equals.  25,427 

1  In  the  spray  kiln  this  is  not  in  reality  lost,  since  part  is  utilized  in  producing  the 
circulation  and  all  the  remainder  is  recovered  in  the  spray  water.  It  is  simply  a  transfer 
of  heat  from  lumber  to  spray  water. 


HUMIDITY-REGULATED  AND  RECIRCULATING  DRY   KILN. 


27 


Compared  to  the  loss  in  the  Forest  Service  kiln,  as  just  shown,  of 
only  897  B.  t.  u.,  this  would  be  enormous.    It  would  mean  an  effi- 

1122 
ciency  of  only  05X27  =  4>41  Per  cent-     ^he  assumption,  however,  that 

it  all  escapes  to  the  outside  air  is  not  carried  out  in  practice  in  moist 
air  kilns,  but  instead  a  large  proportion  of  this  is  returned  by  inter- 
nal circulation,  and  only  a  small  amount  escapes  into  the  air.  It  is 
not  possible  in  the  latter  case  to  calculate  the  theoretical  efficiency, 
since  there  is  no  means  of  knowing  what  portion  of  the  heat  is  re- 
turned in  the  recirculation  within  the  kiln.  The  analysis  is  instruc- 
tive, however,  in  showing  what  enormous  heat  losses  are  possible  in 
a  ventilating  kiln.  In  no  case  can  the  theoretical  efficiency  of  the 
ventilating  equal  that  of  the  Forest  Service  kiln  when  operating 
under  identical  conditions  within  the  drying  chamber. 

INCREASE  IN  DENSITY  PRODUCED  BY  EVAPORATION. 

TABLE  3. — Increase  in  density  of  mixture  of  air  and  vapor  produced  ly  the 
spontaneous  cooling  of  the  mixture  from  the  evaporation  of  moisture  as  it 
through  the  lumber. 


Entering  air. 

After  heating  before 
entering  lumber. 

Leaving  lumber. 

Weight  of  1  c.  c.  of  mix- 
ture in  grams. 

ti. 

hi. 

t». 

hs. 

Dew 

point. 

t3. 

ha. 

Entering  at 

t*j. 

Leaving  at 
tshs. 

0  F. 

Percent. 

0  F. 

P.ct. 

0  F. 

o   p 

Percent. 

32 

100 

158 

1.8 

32 

78.8 

100 

0.0010264 

0.0011658 

32 

100 

158 

1.8 

32 

110.5 

25 

.0010264 

.0011057 

86 

100 

158 

13 

86 

99.5 

100 

.0010126 

.0011094 

86 

100 

158 

13 

86 

140.5 

25 

.0010126 

.  0010394 

140 

100 

158 

64 

140 

140.9 

100 

.0009525 

.0009779 

140 

100 

158 

64 

140 

151.7 

75 

.0009525 

.0010154 

86 

100 

212 

14 

86 

105.8 

100 

.0009310 

.0010915 

86 

100 

212 

14 

86 

146.3 

25 

.0009310 

.0010255 

176 

100 

212 

47 

176 

176.5 

100 

.0007820 

.0008221 

The  weights  are  given  in  grams  per  cubic  centimeter  of  the  mix- 
ture. The  independent  variables  which  may  be  assumed  at  choice 
are  (1)  the  temperature  of  the  entering  air  t±;  (2)  the  relative  hu- 
midity of  the  entering  air  h^;  (3)  the  temperature  to  which  the  air 
is  heated  before  it  enters  the  lumber  t2;  and  (4)  the  degree  of  satu- 
ration of  the  air  leaving  the  lumber,  h3.  From  these,  h2,  t3,  and  the 
volumes  and  weights  of  the  air  and  vapor  are  determined. 

METHOD  USED  IN  CALCULATING  TABLE  3. 

1.  The  temperature,  t"3,  of  the  air  leaving  the  lumber  is  determined 
first,  as  for  Table  1.  The  dew  point  must  also  be  determined  in 
order  to  determine  the  vapor  pressure. 


28  BULLETIN   509,  U.  S.  DEPARTMENT   OF   AGRICULTURE. 

2.  The  following  equation  gives  the  value  of  the  density  (grams 
per  c.  c.  )  of  the  mixture  of  air  and  vapor  : 
j_B—  0.3T8.e       .00129305    * 
760        X  l+.003670t' 

B=  total  barometric  pressure  in  millimeters  of  mercury. 
e=  pressure  of  the  vapor  in  the  mixture. 
t=temperature  Centigrade  of  the  mixture. 
.00129305  is  the  weight  in  grams  of  1  c.  c.  of  dry  air  at  0° 
C.  pressure  760  mm.  under  gravity  at  45°  latitude  and 
sea  level.    The  figure  .003670  is  the  coefficient  of  ther- 
mal expansion  of  air  at  760  mm. 

The  first  fractional  expression  may  be  explained  as  follows: 
Let  di=density  of  dry  air  at  B-e  mm.  pressure. 

dv=  density  of  vapor  at  e  mm.  pressure. 
Then  d=d1-fdv.     The  air  pressure  alone  is  B-e  and 

,     B-e 
di—  a0 


dv=.622  X  d0  X  j. 


when  .622  is  the  density  of  vapor  compared  to  air  at  760  pressure. 


Knowing  the  values  t2  and  t3  and  the  vapor  pressures  at  these  two 
points  (pressures  at  the  dew  points)  the  values  of  d2  and  d3  are 
obtained  from  the  above  equation. 

It  will  be  noted  that  in  every  case  chosen  in  Table  3  the  density 
increases  due  to  the  evaporation,  hence  the  tendency  of  the  air  is  to 
descend  as  it  passes  through  the  pile  of  lumber. 

1  See  Smithsonian  Meteorological  Tables,  Tables  83  to  86. 


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TS     337 


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